1. Field of the Invention
The present invention relates to a television tuner circuit, and more particularly, to a tuner circuit in which the influence of leakage from a local oscillator output and the influence by return loss can be prevented and in which deterioration of distortion characteristics can be reduced.
2. Description of the Prior Art
Generally, the fundamental function of a tuner is to convert a received high-frequency signal into an intermediate frequency. Also, in a frequency conversion operation, the C/N (Carrier to Noise) ratio is so low that power gain is required. In this case, from the aspect of generation of distortion such as cross modulation distortion, it is necessary to control the gains of the several sections of the tuner. However, removal of signal distortion and deterioration of C/N ratio are incompatible. A problem arises as to whether suppression of signal distortion in a tuner and prevention of deterioration of C/N ratio both can be realized.
The problem also arises not only in receivers for a usual television broadcast, but also in the case of CATV broadcast which is in a form of a multi-channel broadcast having many transmission channels.
As an example of the arrangement of channel frequencies for a CATV service, there is a CATV independent broadcast band called a midband. For example, in the Japanese territory, ch.A to ch.I are allotted in the empty frequency band between 3ch and 4ch. In the midband, the frequency of each channel is set so that the relation of frequency band, picture carrier, audio carrier, and color subcarrier frequency per one channel is the same as in a normal channel. As a result, disturbance by a local oscillator output leaked from a receiver against the receivers of other subscribers is eliminated. The same is true in the CATV broadcast on a midband in U.S.A.
In a CATV broadcast, a multi-channel broadcast capability is one of the beneficial characteristics, but a problem arises in that unnecessary signals are generated such as beat signals resulting from the large number of channels. These unnecessary signals promote generation of new unnecessary signals by non-linear distortion caused by the active elements of a tuner.
As described above, noise in a signal obtained through a transmission system may be prevented by suppressing the generation of distortion signals.
FIG. 4 shows a tuner circuit known as a so-called up-down tuner to be used for a multichannel broadcast tuner in a CATV system.
In FIG. 4, a multichannel CATV output is applied to an input terminal 1. Then, it is applied to a first mixer 4 through a band-pass filter substantially formed of a high-pass filter 2 and a low-pass filter 3. The oscillation output of a first local oscillator 6 is applied to the first mixer 4 through an amplifier 5, and the input signal frequency applied to the terminal 1 is converted into a high-pass frequency. The signal converted into the high-pass frequency is input into a second mixer 10 through a band-pass filter 7, an intermediate frequency amplifier 8, and further, a band-pass filter 9. In the second mixer 10, the intermediate frequency signal is converted into a fixed commercial frequency by using an oscillation output of a second local oscillator 11 and the frequency-converted signal is guided out from an output terminal 13 through a band-pass filter 12.
Generally, when non-linear distortion is generated in an amplifier, the input signal voltage and the output signal voltage of the amplifier have the following relation. ##EQU1## where Ye: output signal voltage of amplifier
e: input signal voltage of amplifier PA1 Kn: coefficient showing linearity of amplifier PA1 n: degree of distortion
In the equation (1), non-linear distortion of a fairly high degree may be generated. Practically, however, it is sufficient for design purpose to assume that second order distortion (n=2) or third order distortion (n=3) will occur.
When the distortion shown in the equation (1) is generated in a tuner, cross modulation interference or beat interference is generated. In this case, the cross modulation degree showing the degree of cross modulation interference is in proportion to the square of the amplitude of a signal giving cross modulation interference. This cross modulation distortion becomes very large as the number of transmission channels is increased.
On the other hand, beat interference is generated if distortion components generated in an amplifier exist in the band of a TV high-frequency signal when simultaneously transmitting a number of high-frequency signals.
From the viewpoint of suppressing the influence of amplifier in the tuner circuit shown in FIG. 4, an amplifier is not provided for the input signal of the first mixer 4. Instead, a circuit arrangement is substituted to eliminate the need for an amplifier, which causes distortion. As a result, cross modulation distortion generated in the tuner and distortion due to beat interference are reduced, but this does not improve the C/N ratio (carrier to noise ratio).
Generally, the C/N ratio is expressed by the following equation. EQU C/N (dB)=e.sub.i (dB)-NF (dB)-0.multidot.8 (dB) (2)
Also, the total C/N ratio C/N(m) when the number of cascade amplifiers is taken as m, is determined by the following equation: EQU C/N(m) (dB)=C/N (dB)-10.multidot.log.sub.10 m (dB) (3)
and the total C/N ratio C/N(m) is in inverse proportion to the number of cascade amplifiers m. In other words, when m sets of amplifiers are cascaded, 10.multidot.log.sub.10 m (dB) and the C/N ratio are deteriorated. Accordingly, the input signal level e.sub.i of amplifiers on each portion, which is necessary for holding the C/N ratio obtained when m sets of amplifiers are cascaded to the same value as in one set, is expressed by the following equation. EQU e.sub.i (dB)=e.sub.min (dB)+10.multidot.log.sub.10 m (dB) (4)
In the equation (4), e.sub.min is the minimum signal input level obtained by using the equation (2).
As will be clear from the above, for obtaining the C/N at a fixed value, it is required to set the signal level to a fixed level. That is, it is necessary to set the input/output level to the optimum level considering both of the characteristics of C/N ratio and distortion.
In the tuner circuit shown in FIG. 4, an amplifier is not provided for the input of the first mixer 4, and it is desired that the signal level be low from the aspect of generation of distortion. However, from the view-point of C/N ratio, the signal level necessary to achieve a given C/N ratio by an amplifier on the following stage cannot be obtained, and a problem of deterioration of C/N ratio arises.
For solving the problem, a tuner circuit is shown in FIG. 5 in which an amplifier 14 is interposed between the high-pass filter 2 and the low-pass filter 3 on the input side in the tuner circuit in FIG. 4. In this circuit, deterioration of C/N ratio can be prevented by the amplifier 14, which functions as a preamplifier.
With the amplifier 14 functioning as a preamplifier in the tuner circuit, which is shown in FIG. 5, the signal level is raised up to the input level shown by the equation (4) to obtain a fixed C/N ratio. However, second order distortion and third order distortion problems arise from the non-linear distortion of the amplifier.
Thus, the C/N ratio is improved, but when non-linear distortion exists in the amplifier 14, cross modulation distortion is generated. It is necessary, therefore, to control the gain of the amplifier 14 which functions to improve the C/N ratio so that generation of distortion such as cross modulation distortion is not promoted by a gain of more than required.
When m sets of amplifiers are cascaded, assuming the power gain of each amplifier to be G1, G2, . . . , G(m-1), Gm and the noise figure of each amplifier to be NF1, NF2 . . . NFm, the total noise figure NFt is expressed by the following equation. ##EQU2##
Considered from the stand point of the noise figure and C/N ratio, it is advantageous to control the amplification downstream from the mixer.
However, from the stand point of minimizing distortion it is more desirable to control the amplification on the upstream end of the mixer.
As a result, it is necessary to control the gain of a tuner, that is, to distribute the level of a signal so as not to deteriorate the distortion characteristics or the C/N ratio.
The up-down tuner described above reduces interference of adjacent-channel signals. However, as described above, regarding NF, the gain of an amplifier, which is shown in the equation (5) is regulated by the level distribution to be determined by the equation (4).
Therefore, in the tuner shown in FIG. 4 and FIG. 5, an AGC circuit is required for controlling the gain of the amplifier to maintain C/N ratio characteristics and prevent generation of distortion. A further problem arises as to how to suppress radiation of the output of the local oscillator.